1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer, an ink jet printer and the like. The present invention also relates to an image processing device installed in an image forming apparatus such as a laser beam printer, an ink jet printer and the like.
2. Description of the Related Art
Copying apparatuses and printers using electrophotographic and inkjet methods are well known as conventional apparatuses which reproduce images on papers. Such printers and copying apparatuses reproduce images by adhering, i.e., consuming, toning material such as toner, ink and the like on paper. There are various conditions relating to the black/white ratio of the image, including normal contrast images as in photographs or characters of different thickness or different numbers of characters of the same single page. Accordingly, consumption of toning material fluctuates via the black/white ratio and influences running costs even when reproducing the same single page image. Thus, relatively high contrast results, particularly when reproducing images having a high black/white ratio (mostly black), e.g., when making temporary proof prints is required. In order to reduce such needless cost as much as possible, circuits and apparatuses provided with a toner saving mode have been proposed or realized (e.g., the Laserjet 4 manufactured by Hewlett Packard co., or the EET ASIC circuit made be Destiny, Inc.).
The toner saving mode of the aforesaid conventional apparatuses or circuits reduces the consumption of toning material by approximately halving the number of dots printed by culling every other dot of image data. For example, when printing image data of the character "A" shown in FIG. 1, the print output is as shown in FIG. 2 in the circuit of the aforesaid conventional apparatuses. In this instance, an image entirely of light density is printed as a result, but such printing is efficient for proof printing and when high image quality is unnecessary (e.g., sufficient for reading text).
The aforesaid toner saving mode is an effective function for images having a high black/white ratio. The black/white ratio of a text image is less than 10 percent. In contrast, the black/white ratio of halftone images such as photographs when the entire surface is printed is greater than 30 percent, and a black/white ratio in excess of 50 percent is not exceptional. That is, the toner saving mode can be said to be most effective for halftone images.
In a conventional toner saving mode, however, toner consumption is reduced by culling of simple dot data. Therefore, when accomplishing a culling process on a halftone image expressed by the variable area tone method, the linearity of the tonal expression is lost. This phenomenon is described using FIGS. 3 through 5. The variable area tone method handles submatrices of nxn dots as single image elements, and expresses the variable density of each pixel of a halftone image by changing the percentage of the black pixels within the submatrix. In apparatuses using the variable area tone method, the variable area tone method is accomplished by well known ditheringing methods performed on image data of the halftone image which expresses the variable density of each dot by multi-level data so as to form binary bit image data. FIG. 3 shows a total of 16 ditheringing patterns when expressing the densities of 16 tones of 4.times.4 dot submatrices as single pixels. When these ditheringing patterns are subjected to a culling process which removes every other dot as in the previously mentioned toner saving mode, the result is as shown in FIG. 4 or FIG. 5. Although reducing the number of tones is a natural means to cull dot data within each pattern, the loss of linearity of tone expression can be understood by the changes in density of both patterns of FIGS. 4 and 5 which are nonuniform with many instances of the same consecutive density with abrupt changes to a next density.